<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    function ajax(url) {
      var p = new Promise((resolve,reject) => {
        var xhr = new XMLHttpRequest()
        xhr.open('GET', url, true)
        xhr.onreadystatechange = function() {
          if(xhr.readyState === 4) {
            if(xhr.status === 200) {
              resolve(JSON.parse(xhr.responseText))
            }else if(xhr.status == 404) {
              reject(new Error('404 not found'))
            }
          }
        }
        xhr.send(null)
        
      })
      return p
    }
    ajax('/text.json').then(res => console.log(res))
  </script>
</body>
</html>